Very broadband high power waveguide window



April 18, 1967 J. M. SCOTT 3,315,188

VERY BROADBAND HIGH POWER WAVEGUIDE WINDOW Filed June 2, 1965 APPROX INVENTOR,

JOHN M. SCOTT BY H ATTORNEYS United States Patent 3,315,188 VERY BROADBAND HIGH POWER WAVEGUIDE WINDOW John M. Scott, Fremont, Califi, assignor, by mesne assignments, to the United States of America as represented by the Secretary of the Army Filed June 2, 1965, Ser. No. 460,879

3 Claims. (Cl. 333-98) This invention relates in general to waveguide transmission lines and more specifically to wave permeable gastight partitions therein, called windows. In many high frequency systems it is often found that it is necessary to pass wave energy through a waveguide having portions of its length operating at a reduced pressure or wherein one or more portions of the guide contain different gaseous materials at different pressures. The present invention is useful for providing a very broad band, high power gastight wave permeable window assembly useful in such high frequency systems.

In the hollow waveguide transmission art it has been common practice to insert within the hollow waveguides various types of wave permeable gas-tight windows having a multitude of physical configurations and chemical compositions. For example, such gas-tight windows have taken the form of perpendicular transverse mica walls, ceramic cones, slanted circular ceramic discs, slanted transverse mica walls, and walls having stairstep configurations, to name a few. Universally, these prior art window designs have been plagued with a common problem, namely, discontinuities and irregularities in the guide through which the electromagnetic energy must propagate introduce uncanceled reflections and perturbations in the electromagnetic waves resulting in standing waves or trapped resonant modes thereby decreasing the transmission efficiency and possibly producing excessively high electric fields in the waveguide or elsewhere in the system. The undesirable reflections can arise due to discontinuities in the transmission line or waveguide associated with the junction between window and waveguide as well as from the opposite faces of the window or wave permeable member itself.

A large variety of prior art window configurations exemplify the attempts by the industry to minimize the reflected energy. In general, the approach has involved designing the window and associated junction such that reflecting irregularities or discontinuities are successively arranged along the length of the waveguide in such a manner that a reflection from one irregularity is canceled by the reflection from another irregularity or plurality or irregularities. For example, in the case of thick windows, reflections have heretofore been suppressed by providing a critical thickness of window related to the wavelength, whereby reflections from opposite faces of the window would mutually cancel out. As might be expected, these prior art window designs have been relatively narrow band devices. This is because the irregularities are arranged within the guide to produce reflections that cancel each other out for a given wavelength or frequency. However, for energy of a frequency differing substantially from the central or reference wavelength, satisfactory canceling does not take place resulting in a loss of transmitted power manifested by voltage standing waves in the transmission line between the source and window assembly.

The present invention provides a novel gas-tight window assembly whereby the reflections from various discontinuities and irregularities associated with the novel window and associated waveguide junction cancel out over a very broad band of frequencies. Moreover, the present window design will allow the transmission of relatively large amounts of power therethrough without arcing over or otherwise breaking down.

The principal object of the present invention is to provide a novel improved high power, very broad band gastight window assembly for hollow waveguide transmission lines.

It is another object of the present invention to provide a broad band gas-tight window assembly having a bandwidth equal to or greater than the usable bandwidth of the rectangular waveguides coupled thereto.

It is still another object of the present invention to provide a circular waveguide having secured therein a circular ceramic window and being coupled. to lower impedance rectangular waveguides on opposite sides of said window.

In accordance with the present invention there is provided a high frequency gas-tight permeable window assembly adapted to pass wave energy therethrough over a certain passband of frequencies. It includes a first and second tubular waveguide member and a circular waveguide member intermediate the first and second tubular waveguides. The circular waveguide member is adapted to propagate the TE mode wave energy as the dominant mode and the length thereof is substantially electrical wavelengths long of the cut-off wavelength of the TE dominant mode in the circular waveguide member. In addition, there is included means forming abrupt wavelength transitions connecting the first and second tubular waveguide members to the ends of the circular waveguide member. Also included is a relatively thin gastight wave permeable member disposed transversely of and within the circular waveguide member approximately midway the length thereof.

For a better understanding of the invention, together with other and further objects thereof, referenceis had to the following description taken in connection with the accompanying drawing in which:

FIG. 1 is a perspective drawing, partially cut away of a window assembly embodying the invention; and

FIG. 2 is a longitudinal cross section of the structure of FIG. 1.

Referring now to FIGS. 1 and 2 of the drawing, there is shown one embodiment of the present invention. A section of circular waveguide 10, carries transversely therein a gas-tight permeable ceramic window 12. The permeable window 12 is centered within the circular wave guide 10 and is preferably of a ceramic material having a relatively low dielectric constant. An example of such ceramic material is beryllium oxide (BeO). However, it is to be understood that the window 12 may also be fabricated from other suitable dielectric material such as quartz. Closing off both ends of the circular waveguide 10 are two apertured transversed conductive metallic wall members 14. Two sections of rectangular waveguide 16 are coupled to circular waveguide 10 through the apertured end wall members 14 and are secured to said end walls 14 as for example, by brazing. It is to be understood of course that the propagated rectangular waveguide modes are such as to produce the dominant TE mode in circular waveguide 10.

The apertures in the end wall members 14 represent the abrupt transitions between rectangular waveguide 16 and circular waveguide 10. As shown, the spacing between the abrupt transitions, that is, the spacing between end wall members 14, is made to be electrically in the order of xgc/ 25 where Age is approximately the cut-off wavelength of the TE mode in circular waveguide 10.

The window member 12 is substantially equally spaced from the end wall transitions 14 and is positioned substantially at a situs of maxim-um electrical strength. Also, the thickness of window member 12 is made substantially less than onehalf an electric-a1 wavelength at the center frequency of the wave passband of the wave permeable window assembly.

In operation, energy entering either rectangular waveguide 14 will propagate through the structure. Either 'ectangular waveguide 14 may be utilized as the entry )ort due to the electrical symmetry of the apparatus which nakes for an electrical reciprocal device. The RF energy :ntering either rectangular waveguide 14 will propagate hroug-h the circular waveguide section at frequencies lbove and below the circular waveguide cut-off frequency. it is believed that this is due to the fact that the coupling hrough the circular waveguide section is by means of eakage and fringing fields. The close proximity of the 'ectangular waveguide 14 and the thinness of the ceramic vindow 12 will then cancel each other over a very broad aandwidth. Electrically, the abrupt transitions between 'ectangular and circular waveguide can be represented by lnmatched shunting capacitors C and C shunting a twovire transmission line of a characteristic impedance Z The portions of circular waveguide on either side of the :ircular ceramic window 12 can be represented by corre- ;ponding lengths of higher characteristic impedance Z The relatively narrow portion of circular waveguide 10 which is filled with the permeable window member 12 nay be considered as a short length of circular waveguide of characteristic impedance Z which, in turn, may be cal- :ulated knowing the dielectric constant of the wave permeable window 12. The characteristic of the capacitive dis- :ontinuities between the round and the rectangular waveguides may be measured and the characteristics of the iunc-tion between air filled with dielectric filled round waveguide 10 may be calculated. By using this information approximate dimensions may be arrived at using a Smith chart and standard matching design procedures.

While there has been described what is at present considered to be the preferred embodiment of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is therefore aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. A high frequency gas-tight permeable window assembly adapted to pass wave energy therethrough over a certain passband of frequencies comprising a first and second tubular waveguide member, a circular waveguide member intermediate said first and second tubular waveguide members and adapted to propagate the TE mode wave energy as the dominant mode, means forming abrupt wavelength transitions connecting said first and second tubular waveguide members to said circular waveguide member substantially at the ends of said circular waveguide member, a gas-tight wave permeable member disposed transversely of and within said circular waveguide member approximately midway the length thereof, the length of said circular waveguide member being substantially electrical Wavelengths long of the cut-off wavelength of the TE mode in said circular waveguide member.

2. The assembly in accordance with claim 1 wherein said first and second tubular members are rectangular guides.

3. The assembly in accordance with claim 1 wherein said wave permeable member comprises a ceramic disk.

References Cited by the Examiner UNITED STATES PATENTS 2,958,834 11/1960 Symons et al 333-98 HERMAN KARL SAALBACH, Primary Examiner.

L. ALLAHUT, Assistant Examiner. 

1. A HIGH FREQUENCY GAS-TIGHT PERMEABLE WINDOW ASSEMBLY ADAPTED TO PASS WAVE ENERGY THERETHROUGH OVER A CERTAIN PASSBAND OF FREQUENCIES COMPRISING A FIRST AND SECOND TUBULAR WAVEGUIDE MEMBER, A CIRCULAR WAVEGUIDE MEMBER INTERMEDIATE SAID FIRST AND SECOND TUBULAR WAVEGUIDE MEMBERS AND ADAPTED TO PROPAGATE THE TE11 MODE WAVE ENERGY AS THE DOMINANT MODE, MEANS FORMING ABRUPT WAVELENGTH TRANSITIONS CONNECTING SAID FIRST AND SECOND TUBULAR WAVEGUIDE MEMBERS TO SAID CIRCULAR WAVEGUIDE MEMBER SUBSTANTIALLY AT THE ENDS OF SAID CIRCULAR WAVEGUIDE MEMBER, A GAS-TIGHT WAVE PERMEABLE MEMBER DISPOSED TRANSVERSELY OF AND WITHIN SAID CIRCULAR WAVEGUIDE MEMBER APPROXIMATELY MIDWAY THE LENGTH THEREOF, THE LENGTH OF SAID CIRCULAR WAVEGUIDE MEMBER BEING SUBSTANTIALLY 1/25 ELECTRICAL WAVELENGTHS LONG OF THE CUT-OFF WAVELENGTH OF THE TE11 MODE IN SAID CIRCULAR WAVEGUIDE MEMBER. 